Dying Rainbow Crabs

Question
Hi Karen,

I have been having big problems with my aquarium. I set up the aquarium using 20 litres of tap water and 2 filters.

I bought 2 young rainbow crabs (1.5cm body (not leg) width). They were both very active for the first few weeks then gradually started getting more and more lethargic.

The water would regularly get dirty even though I was emptying 50% of the water every week - 2 weeks. I know they say only empty 20-25% but the water really was getting very dirty.

After about 8 weeks one of the crabs died. I decided at this stage to start testing the water. Ammonia and Nitrite levels seemed ok but nitrate was around 40ppm+. I tested the tap water and its nitrate levels were around the 40ppm mark so I guess my crabs never stood a chance.

I have since started using mineral water instead as the nitrate levels are very very low. However this was too late for my last crab. Within 2 days he lost 6 of his 8 legs and had a moss/fungus growing around his mouth. He died soon after - very distressing watching the little guy to be honest. I have also put a media bag into the tank in the flow of the water that is supposed to control ammonia, nitrate and nitrites. I have also added a little salt as these crabs are supposed to be brackish.

I have always had 2 feeder fish in the tank - the crabs never bothered with them. Since my last crab died I have been testing the water daily and it is close to the 40ppm level again and its only been 5 days since my last water change.

Am I doing something fundamentally wrong? I want to get 2 more rainbow crabs in the next 2 weeks and want to avoid putting them in any danger.

Thanks in advance,

Andy

Answer
Hi Andy,
Although I've never kept Rainbow crabs and my experience only goes as far as Fiddler crabs...According to a little research they appear to be land crabs that do best with a mostly land tank setup with a large dish set in of either brackish or freshwater so they can keep their gills wet. I've read they don't do well when forced to live underwater all the time and I know this is true of Fiddler crabs as well. I'm not sure what your tank setup was like so I'm only making guess but if your setup didn't include places for the crabs to climb out of the water on then that could be a big part of the problem.

A Paludarium setup would probably be great for Rainbow crabs. This goes the same for your fiddlers.Paludariums are part land and part water and they can be a very interesting and cool setup and it is probably your best bet...

Try to use a gravel vacuum on the aquarium when doing water changes. This would help remove a lot of the excess waste and filth in the gravel that is only contributing to your nitrate problem...

If you incorporated some live aquatic or semi-aquatic plants into the system they would further help be excellent nitrate filters. Philodendron is a good semi-aquatic plant but of course there are lots others.

Best of luck!
Karen~